For steeped grains plus malt extract:
The strong bitter is similar to other members of the bitter family (ordinary and best), but with a higher original gravity and alcohol content. This style is sometimes called an extra special bitter due to the well-known commercial example of this style from Fuller's Brewing.
The essential malts are British pale ale (Maris Otter is popular) and crystal malt around 4 to 7%. Other tweaks can include special roast, victory, biscuit, or aromatic. Sugar can be used (5 to 10%), although Zainasheff says that he usually avoids sugar.
British hops to consider are Goldings, Fuggle, Challenger, Target, Willamette, and Northern Brewer. Zainasheff's preferred late hopping schedule is one hop addition at the end of the boil. Aim for a BU/GU near .68.
Popular yeasts to try are Fermentis S-04, Lallemand Nottingham, White Labs WLP002, and Wyeast London Ale 1968.
Beer-analytics.com strong bitter
Zainasheff, J. (2008, March-April). Extra special bitter: (a.k.a. English pale ale). Brew Your Own, 19 - 22.